1. Field of the Invention
This invention relates to improved silver catalysts for the production of ethylene oxide, their preparation, and their use in ethylene oxide processes.
2. The Prior Art
Materials consisting of silver upon supports are known to be useful catalysts for the production of ethylene oxide by the controlled incomplete oxidation of ethylene with molecular oxygen. A great variety of modifications have been proposed to improve the activity and selectivity of these catalysts. These modifications have involved, for example, the supports employed, the method of production, the physical form of the silver on the support and the addition of additives to the catalyst.
The alkali metals and their salts have been repeatedly proposed as additives for various silver ethylene oxide catalysts. Carter, in U.S. Pat. No. 2,125,333, issued Aug. 2, 1938, was among the first to disclose alkali metal addition. He specified the use of "small amounts" of alkali metals, including both sodium or potassium, in his silver catalyst. Later patents elaborated on this disclosure, but often with contradictory teachings. McNamee et al in U.S. Pat No. 2,238,474, issued Apr. 15, 1941, disclosed that while 100 ppm by weight to 24% by weight of sodium improved silver catalysts, these amounts of potassium had a detrimental effect in catalyst performance. Sears, Jr. et al in U.S. Pat. No. 2,615,900, issued Oct. 28, 1952, cites a large number of promoters useful in broad weight ranges, but makes no distinction in the effectiveness of the various promoters. Sacken, in U.S. Pat. No. 2,671,764, issued Mar. 9, 1954 disclosed the use of large amounts of alkali metal sulfates. Sacken in U.S. Pat. No. 2,765,283, issued Oct. 2, 1956, proposed adding from 1 ppm by weight to 2,000 ppm by weight of an inorganic chlorine compound to the catalyst support prior to the addition of silver to improve the finished catalyst. Saken's inorganic chlorine compounds included the alkali metal salts of chlorine acids, especially sodium chloride. Carlson et al in U.S. Pat. No. 2,773,844, issued Dec. 11, 1956 discloses a multistep silver deposition process but is not concerned with simultaneous deposition of alkali metals. Gould et al, in U.S. Pat. No. 2,799,687, issued July 16, 1957, disclosed that when from about 20 ppm by weight to about 1.6% by weight of inorganic halide (sodium chloride or preferably potassium chloride) are added as separate solid particles to a fluidized bed of supported silver catalyst, the halide acts as a suppresant, inhibiting the catalyst activity. Hosoda et al in U.S. Pat. No. 3,144,416, issued Aug. 11, 1964, cites a number of promoter materials, but gives no limits on their concentration. Kriger et al in U.S. Pat. No. 3,563,913 issued Feb. 16, 1971, generally discloses the use of alkali and alkaline earth metals as promoters, listing specifically lithium with no reference to cesium, rubidium or potassium. He notes that these promoters are preferably added to the catalyst support before the latter is impregnated with the silver compound containing solution. Long in U.S. Pat. No. 3,575,888, issued Apr. 20, 1971, disclosed the use of aluminum oxide supports having a pore volume between about 0.15 and 0.30 ml/gm and surface area below about 10 m.sup.2 /gm. Nielson in U.S. Pat. No. 3,702,259, issued Nov. 7, 1972 uses certain organic amine solubilizing/reducing agents to produce uniformly spaced, adherent, hemispherical deposits of metallic silver on catalyst supports.
The prior art clearly recognizes that alkali metal compound addition changes, for better or worse, the character of a silver ethylene oxide catalyst. There is no recognition of real advantages for any special amounts of alkali metal addition, 1 ppm being considered the equivalent of 2,000 ppm, for example. Moreover, there is no recognition of any special chemical advantages for the addition of certain alkali metals over other alkali metals. Further, there is no recognition that the simultaneous deposition of the alkali metal with the silver gives any special advantage over a sequential deposition scheme.